1. Field of the Invention
The present invention relates to a polarizing optical element and a display device including the optical element.
2. Description of the Related Art
An absorbing dichroic polarizer is currently used most extensively among various other types of polarizers. The absorbing dichroic polarizer is obtained by adsorbing a compound with light absorption anisotropy (e.g., iodine or a dichroic dye) onto a stretched polymer film and orienting the compound. This polarizer decomposes incoming light into polarization components that cross each other at right angles, absorbs polarization components that are parallel to the absorption axis of the dichroic dye and transmits polarization components that are perpendicular to the absorption axis. Accordingly, the light that has been transmitted through the polarizer becomes linearly polarized light. However, if such an absorbing polarizer is used in a display device, then the transmittance to natural light can never exceed 50% in principle, thus resulting in low optical efficiency.
To minimize the loss caused by the absorption of light into such an absorbing dichroic polarizer, another type of polarizer, which reflects linearly polarized light with a particular polarization direction and transmits polarized light with the other directions, was developed and already used actually. Such a linearly polarized light reflecting polarizer is a stack of non-absorbing dielectric materials (e.g., obtained by alternately stacking two different types of polymers A and B as ABABA . . . ). Such a polarizer is formed by alternately stacking the two types of materials A and B and then extruding them together. Accordingly, the stacked materials are stretched along one axis (which will be referred to herein as an “x-axis”) but are hardly stretched along the other axis (which will be referred to herein as a “y-axis”). This is why the polarizer exhibits high reflectance to the linearly polarized light that is polarized in the x-axis direction and high transmittance to the linearly polarized light that is polarized in the y-axis direction. If that stack is designed such that the thicknesses of the respective layers change vertically, a reflective polarizer, exhibiting high reflectance to visible light in a broad range, can be obtained.
A polarizer with a micro metal grating (wire grating) is another known type of linearly polarized light reflecting polarizer (see Japanese Laid-Open Publication No. 9-90122, for example). This polarizer has a structure in which metal wires made of the same metallic material are arranged parallel to each other. The diameter of the respective metal wires is sufficiently smaller than the wavelength of the incoming light. This type of wire grating polarizer has polarization properties of reflecting polarization components that are parallel to the metal wires (i.e., a TE wave) and transmitting polarization components that are perpendicular to the metal wires (i.e., a TM wave).
When a reflecting polarizer is used in a display device, the polarizer exhibits an identical reflectance to the same polarized light ray that enters the upper surface and the lower surface of the polarizer, thus increasing the optical efficiency. However, if this polarizer is used by itself in a display device, dark display is not realized due to low light attenuation and therefore the contrast decreases on the screen. To achieve the dark display, the display device needs to include the reflecting polarizer and the absorbing polarizer in combination. In that case, however, the number of required components and the manufacturing cost both increase. In addition, the thickness of the display device also increases by the thickness (of about 100 μm) of the absorbing polarizer.